1. Field of the Invention
This invention relates to the field of data error concealment.
2. Description of the Prior Art
When digital image or video data is recorded or transmitted it can be corrupted by data errors. For example, portions of data recorded on and then replayed from a magnetic medium may suffer errors due to medium defects or dirt particles on the medium's surface. As video data is usually processed in real time, it is not generally possible to re-read or re-transmit the corrupted data.
To counter this problem it is known to encode data before recording or transmission using error correcting codes. This involves adding extra digital information, derived from the data, to form a composite data stream. The additional information is such that certain errors in the original data can be detected and the correct value of the data element in error calculated and reinserted. However, when an error correcting code is used, a balance must be struck between the requirement to be able to correct commonly occurring errors and the increased redundancy or data overhead caused by adding the error correcting capability. This generally places an upper limit on the length or frequency of data errors which can be completely corrected, although the system may allow the presence of more serious, uncorrectable, errors to be detected.
In contrast to an analogue system in which steadily increasing degradation generally has a steadily worsening effect on a video signal, if the degradation of a digitised image or video data stream exceeds the threshold at which the errors can be corrected, then the effect on the picture reconstructed from the data can be sudden and subjectively very noticeable. In order to reduce this problem it is known to use error concealment to mask the subjective effect of uncorrectable data errors on the reconstructed picture.
Error concealment in these extreme situations exploits the inherent redundancy present in digitised images or video signals, in that an attempt is made to estimate or interpolate a replacement value for a picture element (pixel) from surrounding pixels in the same picture or from corresponding pixels in previous and subsequent frames. If this is not possible because the surrounding pixels have themselves suffered data errors, then a last resort approach may be to re-use corresponding pixels from an earlier frame to conceal the missing pixels. An example of an error correction and concealment system is that used in the D1 Digital Video Tape Recorder, described in the book `Introduction to the 4:2:2 Digital Video Tape Recorder` (S. Gregory, Pentech Press, 1988).
A recently adopted signal processing technique is to decorrelate digital image or video data into respective frequency components in the two-dimensional spatial frequency domain. This technique has a number of advantages, particularly relating to the ease with which the resulting data can be compressed for storage or transmission.